Sound reproducing system



Sept. 26, 1933.

E. D. cook 1,928,410

SOUND REPRODUCING \SYSTE'M Filed Feb. 16,1931 4 Sheets-Sheet 1 ll Z O '5 L m g r' ij-wuamtoz $51 hut a t Mme W Sept. 26, 1933. E. D. cooK 1,928,410

' SOUND REPRODUCING SYSTEM Filed Feb. 16, 1931 4 Sheets-Sheet 2 lwvwvv FIG.4

Qvwemtoz [/13 warm 0. (00k 11.4.5, GHQT/"(Mg Sept. 26, 1933. E. D. CO OK 1,928,410

SOUND REPRODUCING SYSTEM Fi1ed.Feb. 16, 1931 4 Sheets-Shet s Qvwemboz f/lsworf/l D. (00k 4 w L r W M. M n Y M @337 w I 6 h wwnh F M s n w K T W m ij 1 J. all w Sept. 26, 1933. v E. D. cooK 1,928,410

SOUND REPRODUCING SYSTEM Filed Feb. 16, 1931 I 4 Sheets-Sheet 4 99 l l I0] 91 I00 llll- M FIG. 7

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Patented Sept. 26, 1933 SOUND REPRODUCIN G SYSTEM Ellsworth D. Cook, Flushing, N. Y., assignor to United Research Corporation, Long Island City, N. Y., a corporation of Delaware Application February 16, 1931. Serial No. 516,084

10 Claims.

This invention relates to sound translating systems, and particularly to means for improving the quality of reproduction in a sound system by masking incidental interfering sounds, and by modifying the volume level range of reproduced sound.

In sound translating systems generally, the quality of the translated sound has been injured by the presence of extraneous noises of various sorts, such as record scratch or needle noise from mechanical records, photo-cell noise, surface noise and grainyness sounds in photo-electric recording, static and tube noises in other sound systems, as well as line noises, amplifier noises, and other sources of interfering sounds.

The presence of such extraneous sounds in any sound translating system limits the minimum sound level at which reasonably satisfactory reproduction can be obtained to a value sufficiently great to obscure the incidental noises, and since a limit is always imposed upon the maximum sound volume by the limits of the record material, the usable sound volume level range is undesirably restricted.

An object of this invention is to reduce the apparent magnitude of incidental noises in a sound translating system.

Another object is to increase the range of sound volume obtainable from a sound translating system.

Still another object is to modify the amplifying power of an amplifier system in accordance with the sound level of a sound transcription passing therethrough.

A further object is to reduce the amplifying power of an amplifying system during intervals of low volume level therethrough, and to increase the amplifying power of an amplifier system during periods of high volume level passing therethrough.

In the prior use of sound translating systems difficulty has been experienced from the presence of random and extraneous sounds and noises which have interfered with the low sound levels, being objectionably obtrusive during periods of soft tones. This invention provides a means whereby during periods of low or zero sound level, the amplifying power of the amplifier is reduced so that the extraneous noises from such sources as needle scratch, static, tube and line noises, etc. are amplified by only a very small amount, causing them to be much less perceptible to the loud speaker than under normal amplifier conditions. Under conditions of normal tone volume the amplifier may then function at the usual constant; and if desired, at high sound volume levels the amplifier constant may be increased. Thus not only is the obtrusiveness of incidental sounds greatly diminished but the system may serve also as a means for increasing the sound volume range from a sound translation.

The system of this invention may be used with a mechanical record, with a photo-electric record, or with any type of transmitted sound translation. It is particularly adapted to the reproduction of mechanical phonograph records. It may likewise be used both for the recording and reproducing of sound records generally. That is, in the case of a sound production where the sound volume ranges are undesirably high for the limits of the record material to be used, the system of this invention may be employed to reduce the sound volume range upon the record, and it may, thereafter, be used to increase the sound volume range during the reproduction of the record to one approximating that of the original production. Similarly when sound events are produced specifically for recording, and the sound volume range is reduced by the conductor in the usual way for the recording, the volume range may be increased by the system of this invention. And furthermore in any of these suggested methods of using the system of this invention, the obtrusiveness of scratch sounds and other extraneous noises is greatly reduced.

The objects of this invention are thus obtained by a system combining an amplifier device and means for modifying the amplification factor thereof in accordance with the amplitude of the sound transcription transmitted therethrough.

Other objects and structural details of the invention will be apparent from the following description when read in connection with the accompanying drawings, wherein:

Fig. 1 is a diagrammatic representation of circuits and apparatus of an embodiment of the invention; and

Figs. 2, 3, 4, 5, 6, 7 and 8 are alternative embodiments adapted to perform the same functions.

Referring to Fig. 1, a reproducer 1 is provided adapted to cooperate with the transcription or sound record, which is not shown, but may be a mechanical sound record, a photo-electric sound record, or a radio or wire transmission, or other source of signals corresponding to sound. The output from the source 1 is transferred through leads 2 and 3, through a transformer 4, to a vacuum tube 5, having an output transformer 6 and a source of plate current as shown. The grid circuit of the tube 5 includes a grid bias battery '7, resistor 8, and a volume control resistor 9. The output from the tube 5 through the transformer 6 is transferred to a main amplifier 11 and a loud speaker 12.

An auxiliary vacuum tube 14 is provided and the grid circuit thereof connected to the vibratory current source 1 in parallel with the grid circuit of the tube 5. The grid circuit of the tube 14 is provided with a grid condenser 15 and a grid leak 16. A plate battery 17 is also provided for the tube 14. The resistor 9 is connected both in the grid circuit of the tube 5 and in the plate circuit of the tube 14 as shown.

In the operation of this embodiment, the bias upon the grid of the tube 5 is the sum of the voltages from the battery '7 and the voltage drop across the resistor 8 produced by the plate current of the tube 14 flowing therethrough. The tube 14 being equipped with grid leak 16 and condenser 15 it acts as a detector in the presence of oscillations in its grid circuit, and reduces the current flow in the plate circuit. Under conditions of low sound level from the source 1, very little signal is detected in the tube 14 and accordingly its plate current remains at a relatively high value giving a substantial voltage drop across the resistor 8 and applying a high value of bias voltage to the grid of the tube 5, thereby reducing its amplification constant to a minimum value. In consequence only a small portion of the incidental noise from the pickup 1 is transferred to the main amplifier 11. As the vibratory output of the source 1 increases, a greater amount of signal is detected by the tube 14, and in consequence the plate-circuit current is reduced, reducing the drop across the resistor 8 and thereby the grid bias in the tube 5, thus increasing the amplifying power of the tube 5, and increasing proportionately the amount of energy transferred to the amplifier 11 and speaker 12.

In this way the ratio between the amount of energy in the input circuit of the tube 5 and the output circuit is varied according to the actual value of signal level in the input circuit, thus greatly reducing the amount of needle scratch noise or other incidental noises transferred to the loud speaker 12 during periods of low sound volume. It is, of course, obvious that this system is applicable to any form of amplifier operating over a range of sound volume levels in the presence of incidental and extraneous noises. Furthermore the system has the additional possible advantage of increasing the sound volume range of a sound transcription, which is a desirable feature in view of the customary relatively narrow range of sound volume levels in the usual sound transcription.

The system of Fig. 1 utilizes a grid leak and grid condenser with the detector tube 14. Plate circuit detection by means of a high grid bias may also be used in the tube 14 in a similar way, as is shown in Fig. 2 in which the same source 1 of signal is connected by leads 2 and 3 through a transformer 4 to the tube 5. In this embodiment a bias battery 19 is connected in the grid of the tube 14 to give the necessary grid bias for detection therein and the leads to the resistor 8 from the plate circuit of the tube 14 are reversed, since upon detection by grid bias, the plate current rises instead of falling, as in grid leak.

In the operation of this device the grid battery 7 of the tube 5 provides a grid bias sufficiently high to reduce the amplifying power of the tube 4 to a minimum value. Thus during periods of low sound volume the extraneous noises are amplified by a minimum amount and their volume in the loud speaker 12 is at a relatively low value. When vibratory energy is delivered from the source 1 a portion of it is detected by the tube 14, producing an increase in current flow in the plate circuit therein. The voltage drop across the resistor 8 is thus in opposition to the voltage produced by the battery 7 and as it increases with the increased current flow in the tube 14 the bias upon the grid of the tube 4 is reduced, thus increasing its amplifying power and delivering a proportionately larger amount of energy to the amplifier 11 and loud speaker 12.

Alternatively, the system of Fig. 3 may be utilized. In this figure a pickup 21 of any desired form may be utilized connected by leads 22 and 23 to an amplifying transformer 24, through by-pass condensers 25 and 26. A vacuum tube 27 is shunted across the leads 22 and 23 in parallel with the primary coil of the transformer 24 and supplied by a plate battery 28 in series with a resistance 29. An auxiliary amplifier system 31 is supplied by leads 32 and 33 connected in parallel with the leads 22 and 23 to the reproducer 21. The output of the amplifier 31 is connected by leads 34 and 35 between the cathode and grid of the tube 27. The output of the transformer 24 is supplied by appropriate leads to a main amplifier 38 feeding a loud speaker 3'7.

In operation, the vacuum tube 27 provides a variable shunt across the input to the transformer 24. The effective resistance value of this shunt is determined by the grid potential of the tube, which in turn is set by the detector effect of the combination of grid condenser 38 and grid resistance 39. Accordingly, the vibratory voltage output from the pickup 21 is partially conveyed to the transformer 24 by way of the leads 22 and 23 and from the transformer through the amplifier 36 to the loud speaker 3'7. During periods of low sound reproduction, the impedance of the tube 27 is at a minimum value, and accordingly a substantial portion of the signal energy is shunted from the transformer 24, resulting in a lowered sound level in the amplifier and speaker system. When, however, the signal level from the pickup 21 rises, a greater volume level is applied through the amplifier 31 to the grid circuit of the tube 2'7, and accordingly the impedance of the tube 27 is raised, resulting in a lowered shunting effect, and the transfer of a larger proportion of the signal energy through the amplifier 36 to the speakers. This system thus likewise reduces the amount of extraneous noises present in any type of sound reproduction especially at low sound levels, and also may be used for the reproducing of any desired type of record at an expanded sound volume range, and by obvious changes it may similarly be utilized to contract the sound range for recording purposes.

As previously suggested, the system of this invention may be utilized to contract the sound level range as well as to expand it, thus producing a complete system. For this purpose the embodiment of Fig. 4 may be utilized in which a microphone 41 is placed in a proper position with respect to the sound to be transcribed. The output from the microphone 41 may be supplied through leads 42 and 43 and a transformer 44 to a vacuum tube 45, which in turn is connected to the primary of a transformer 46. A plate battery 47 is provided as before, and a grid battery 48, a grid circuit resistor 49 and a volume control resistance 50. The secondary of the transformer 46 is connected to a power amplifier 51 and a recording device 52 cooperating with a record driving member 53. An auxiliary tube 54 is provided as before with the grid circuit connected to the circuit of the microphone 41, which includes a grid condenser 55 and a grid leak 56. The plate battery 57 is provided as before. It will be noted that this system corresponds closely to that of Fig. 1 except that the plate circuit leads of the tube 54 are reversely connected to the resistance 49.

In the operation of this device, the bias upon the grid of the tube 45 is the sum of the voltage produced by grid battery 48 and the voltage drop from the current in the resistor 49. As indicated these voltages are in opposition. They may be so adjusted that during periods of low sound volume, the bias upon the grid of the tube 45 is adjusted to the value for optimum amplification. Thus low sound levels received by the microphone 41 are amplified by a maximum amount for recording by the cutter 52 at a substantial vibration level. As the sound level rises, however, the detector effect of the tube 54 reduces the current in its plate circuit, reducing the voltage drop across the resistor 49, thus allowing an increase in the grid bias upon the tube 45, reducing its amplifying power, and thus reducing by a proportionate amount the amplitude at which the larger amplitude vibrations are recorded. It is, of course, obvious that for a satisfactory record, the louder sounds are recorded at a higher amplitude level than the softer sounds, but at an amplitude level which is not as high in proportion as the amplitude level of the original sounds. This system thus produces a contraction of the amplitude level range in the record.

Under ordinary recording conditions such a system is not needed since the performers can be directed by the conductor to produce a relatively high sound volume in soft passages and correspondingly to avoid the excessive volumes which might be used on a concert stage for the loud passages. The system is, however, of particular advantage for the recording of performances which are not produced in the studio but under conditions of maximum volume range as in the usual public performances.

As before, this system is of substantial value in minimizing the effect of extraneous noises either in the room in which the production is performed or from the various electrical features of the recording system, since it gives in the soft portions of the production sufficient volume level to aid in masking the presence of extraneous sounds.

The record produced by the cutter 52 may then be played by the reproducer 1 by the system shown in Fig. 1, whereupon the volume range obtained from the speaker 12 is substantially the same volume range as the original production, and is substantially free from extraneous noises during low volume portions.

The system of Fig. 3 may similarly be used for recording at reduced volume range as shown in Fig. 5 by replacing the reproducer 21 by a microphone 61, replacing the loud speaker 37 by a cutter 62 working on a recording table 63, and replacing the grid condenser 38 and grid leak 39 by a bias battery 64, other members remaining the same as shown and described in connection with Fig. 3. In the operation of this embodiment the tube 27 has a minimum conductivity during periods of low sound volume in the microphone 61, and accordingly a minimum amount of the sound energy is shunted from the transformer 24, and therefore sound of low volume is recorded at a fairly high vibration level. Because of the bias detection characteristics of the vacuum tube 27, an increase in signal level from the microphone 61 increases the conductivity of the tube 27, thereby increasing its shunting effect, and by-passing a larger portion of the vibratory energy, thus diminishing the extent of the increase of amplitude at which the vibrations are recorded on the record table 63. This system thus functions in a manner similar to the system of Fig. 4 for the production of records of restricted vibration-amplitudelevel range. These records similarly may be reproduced by the usual reproducing system or by the reproducing systems herein disclosed for the production of greater volume range in the resulting sound.

In the embodiment of Fig. 6, a pickup 71 is connected through balanced transformers 72 and 73 to an amplifier system 74 and a speaker 75. The transformer 72 has a single coil primary, and a double secondary coil, the respective halves of the secondary being wound in opposite directions. The transformer 73 also has a double coil primary, and a single coil secondary adapted to be connected to the amplifier system 74. The respective double coils of the transformers are connected together as shown, by midpoint leads, which are connected together through a resistance 76, and the appropriate batteries shown. One of the secondaries of the transformer 72 has its second terminal connected to the grid of a vacuum tube 77, the plate of which is connected to the second lead of the correspond ing coil in the transformer 73. The second lead of the second coil in the secondary of the transformer 72 is connected through a blocking condenser 78 to the corresponding terminal of the second primary of the transformer 73.

An auxiliary amplifier tube 79 is provided, with the grid thereof connected to the pickup 71, in parallel with the primary coil of the transformer 72. The output of the tube 79 is passed by means of a transformer 81 to the grid circuit of a detector tube 82 equipped with the grid condenser 83, and grid leak 84. The plate circuit of the tube 82 likewise contains the resistance 76.

In the operation of this device, oscillatory voltages from the pickup 71 are supplied to the transformer 72, and'by it to the transformer 73. The phases of the respective portions are, however, in opposition and the gain of the amplifier tube 77 is partially or substantially neutralized by the resistances 85 and 86. Accordingly little or no energy is delivered from the transformer 73 to the amplifier 74 during periods of low or zero sound volume. Simultaneously during low sound volume a small amount of energy only is received by the tube 82, and accordingly its plate current is relatively high, and a relatively high current flows in the resistance 76, applying a relatively high bias to the tube 77 and reducing its amplification factor. When, however, the sound level from the pickup 71 rises, the detecting effect upon the oscillations applied to the tube 82 causes a decrease in the plate current therein, a corresponding decrease in the voltage drop in the resistance 76, a reduction in the bias upon the grid of the tube 77, and accordingly an increase in the amplification factor, which serves to unbalance the energy in the respective primary coils of the transformer 73, and produces an output therefrom to the amplifier 74 and speaker 75. This effect rises with an increase in sound level in the pickup 71 to produce a proportionately greater sound level in the output.

The disclosure of Fig. 6 utilizes a grid leak and grid condenser detector system with the tube 82. Alternatively plate detection as by the use of a grid bias from a suitable bias battery may [ill be utilized, in which case the polarity of the leads to the resistance 76 from the plate circuit of the tube 82 must be reversed.

Still another embodiment is shown in Fig. 7 in which the vibratory signal source may be an electric pickup member 91 from which the vibratory voltage output is supplied through leads 92 and 93 to a bridge system 94 having resistance arms 95, 96 and 97 and a vacuum tube 98 for the fourth arm. The resistance arms are adjusted in size with respect to the plate circuit impedance of the vacuum tube 98 so that the bridge is nearly balanced in the absence of signals. The leads 92 and 93 are connected across one diagonal of the bridge, and a plate battery 99 and output resistance 101 are connected in series across the other diagonal, to which are also connected output lead to an amplifier 104 and a loud speaker 105. Auxiliary leads 106 and 107 are connected to the pickup 91 in paral lel to the leads 92 and 93 and convey a portion of the output current from the pickup 91 to an auxiliary amplifier 108, the output of which is connected by leads 109 and 110 to the grid circuit of the triode 98. The grid circuit is in addition provided with a grid condenser 102 and grid leak 103.

In operation the record is translated by the reproducer 91 into electrical vibrations, which are conveyed by the leads 92 and 93 to the bridge 94. At low or zero sound volume the bridge is substantially in balance, and accordingly a very small amount of vibratory energy is conveyed by the leads 92 and 93 to the amplifier 104. Since during low or zero sound volume of the original production, mainly scratch noises are delivered from the pickup 91, substantially none of such sounds are transmitted to the amplifier 104, and accordingly are not reproduced from the loud speaker 105. As the amplitude of the record increases, however, the output of the reproducer 91 increases, a portion of the output is conveyed to the auxiliary amplifier 108, and from it to the tube 98, through the grid condenser 102 under the influence of grid leak 103. The leak and condenser cause the tube to act as a detector and its plate impedance is changed in accordance with signal level, thereby unbalancing the bridge 94 by an amount corresponding to the amplitude of the record. Accordingly, as the record ampliiude increases, a proportionately greater amount of signal voltage is delivered to the amplifier 104, and an increasingly greater sound output obtained from the speaker 105.

This system, as before, is shown as utilizing a grid leak and grid condenser for detection of the frequencies received. Alternatively, of course, the grid leak and grid condenser may be replaced by a grid bias battery of suitable size to give plate circuit detection, in which case the relation of the resistances may be slightly modified in a manner which will be obvious.

Another embodiment is shown in Fig. 8. In this embodiment a pickup 111 supplies electrical vibrations corresponding to the record sound to the primary of a transformer 112, the secondary of which is connected to the grid and filament of a vacuum tube 113. An output transformer 114 is connected in the plate circuit of the tube 113 and supplies vibratory energy to an amplifier and speaker 116. Connected to the secondary of the, transformer 112 and in shunt to the tube 113 is a second vacuum tube 117, serving as an amplifier, and feeding, through a transformer 118, a second vacuum tube 119. All of these tubes are provided with filament heating batteries and circuits as indicated. A bias battery 121 serves to bias the grids of the tubes 113 and 117 to the mid-points of the characteristic curve for accurate repeatering and good reproduction. The plate circuit of the tube 113 is equipped with a plate battery which may be regarded as consisting of two sections, 122 and 123, the plate circuit of the tube 119 being supplied from a tap at the junction of the two batteries. The tube 119 is provided in its grid circuit with a bias battery 124 sufliciently large to provide plate circuit detection of the audio frequency oscillatory energy. The tube 113 is also provided with a potentiometer and battery system 125 for further adjusting the bias upon the grid. The plate current of the tube 119 is supplied from the battery 123 by way of the tube plate, cathode, a lead to the secondary of transformer 112 and the battery and potentiometer resistance system 125 to the cathode circuit of tube 113 and the negative pole of the battery 119. Accordingly a change in current through the tube 119 changes the voltage drop in the potentiometer resistance and therefore the bias upon the tube 113 and thereby changes the effective circuit amplification constant.

In operation at low signal volumes, a portion of the pickup energy is transmitted to the tube 113 amplified therein, and transferred through 105 transformer 114 and amplifier 115 to the speaker 116. Another portion of the pickup energy is amplified by the tube 117, and integrated by the tube 119 to produce a change in the current of the plate circuit thereof and accordingly in the potentiometer resistance 125. This change in current is in such a sense as to increase the bias upon the grid of the tube 113, thereby reducing its amplification constant and reducing the amount of vibratory energy transferred to the 115 amplifier and speaker. When the volume increases, however, the increase is transferred by the tube 117 to the tube 119 and integrated thereby to produce a change in plate current. This changes the current fiowing in the resistance 125, and the voltage drop therein and accordingly changes the bias upon the tube 113. This change is readily made to occur in such a direction as to reduce the bias on the tube grid and increase its amplifying constant, whereupon a proportionately larger amount of pickup signal is transferred through the transformer 114 and amplifier 115 to the speaker 116. This change may be made cumulative to spread the range of sound volume levels.

The above disclosed system may effect the amplifying properties of the associated amplifier system over the whole volume level range. It is not, however, necessary that the whole volume level range be modified, but instead the change in amplifying properties may be confined to a portion only of the volume level range. For instance, if the tube 14 of the embodiment of Fig. 1 is small, or is provided with only a small power source 17, the change in plate current produced by signals in its grid circuit from the pickup 1 may be small, and may be effective over a small portion only of the total volume range. That is, in the embodiment of Fig. 1 if the battery 17 is small, and the input from the pickup 1 is relatively large, the total available change in plate current in the tube 14 may occur over a volume level range from the lowest pickup level to a moderately low pickup level giving the 150 effect of changing the amplification range over the low volume levels, but leaving the intermediate and high volume levels unaffected. This system thus serves as a scratch filter alone; and is effective only during intervals of the lowest sound levels.

Similarly, by appropriate adjustment of the tube 14 and its circuits, the effect may be made to act only upon the high levels, altering the highest volume levels only.

The system may by appropriate modifications be caused to function in stages, causing an alteration in amplification at the low volumes to remove extraneous sounds, leaving the intermediate volume levels substantially unchanged, and acting to modify the highest sound levels, which modification may be either a reduction or an increase.

This invention thus provides systems for increasing the range of sound volumes obtainable from a sound record by a plurality of systems utilizing balance methods and change of grid bias upon amplifier tubes. Obviously other modes of operation are possible such as variation of plate battery voltage or filament temperature but with existing apparatus, these have certain limitations.

While there are shown herein several embodiments of the device of this invention, it is possible to produce still other modifications thereof without departure from the inventive concept therein, and it is therefore desired that only such limitations shall be imposed upon the appended claims as are stated therein, or required by the prior art.

The invention claimed is:--

1. In a sound record system a bridge device, a source of oscillatory electrical energy, corresponding to sound vibrations, a transcribing device, and means for controlling the balance of the bridge according to the vibration amplitude of the electrical vibrations.

2. In a sound record system, a microphone, a bridge member connected thereto, an amplifier therefor, a recording system connected to said bridge, and means connected between said amplifier and said bridge for varying the balance thereof according to the sound level at the microphone to produce a record of reduced volume range, and a reproducing system adapted to cooperating with such a record comprising a reproducer member, a second bridge system, an amplifier and loud speaker, and means connected between said reproducer and said bridge for controlling the balance of said bridge according to the amplitude of said record to expand volume range.

3. In a sound system, a source of audio frequency electric impulses, an amplifying means therefor, means for conveying impulses from said source to said amplifier and means also connected to said source adapted to control the amplifying properties of said amplifier over'low impulse amplitude levels only.

4. In a sound system, a source of audio frequency electric impulses, an amplifying means therefor, means for conveying impulses from said source to said amplifier and means also connected to said source adapted to modifythe amplifying properties of said amplifier at substantially zero input levels to prevent amplifications of background noises.

5. In a sound record system, a sound record having portions of variable sound level, some thereof being nearly silent, and means for the reduction of noise during such silent portions, comprising an amplifier in said reproducing system and an auxiliary means connected to said amplifier adapted to reduce its amplification factor during said silent portions of said record.

6. Arrangement for reproducing sound from a record thereof which comprises means for translating the sound record into electrical variations, an amplifier therefor for reproducing said variations in amplified form and means responsive to electrical variations generated during silent periods of said record for controlling the grid potential of said amplifier.

7. The method of reproducing sound from records thereof which comprises translating the sound record into electrical variations, amplifying said electrical variations, generating other electrical variations during silent periods of said record and in varying said amplification in accordance with said other electrical variations.

8. The method of reducing surface noises in the reproduction of sound from grooved records which comprises translating said records into electrical variations, amplifying said variations and reducing the degree of said amplification in response to and under control of said variations which are of low amplitude comparable to the amplitude of electrical variations produced by surface noises from said record.

9. The combination of means for supplying electrical variations corresponding to sound waves in natural relative amounts of high and low frequencies free from volume compression of one frequency range with respect to another frequency range, an amplifier therefor, having a cathode, an anode and a grid, means for supplying a comparatively large bias voltage to the grid of said amplifier for rendering said amplifier substantially ineffective during low volume of said electrical variations comparable to noise currents and means responsive to electrical variations of a value larger than noise currents for reducing said grid bias to permit said amplifier to substantially amplify said electrical variations in natural relative amounts of high and low frequencies substantially free from volume compression or expansion of one frequency range with respect to another frequency range.

10. The combination of means for supplying electrical variations corresponding to sound 'waves in natural relative amounts of high and low frequencies free from volume compression of one frequency range with respect to another frequency range, a translating device therefor, means for supplying a comparatively large bias voltage to said device for rendering said device substantially ineffective during low volume of said electrical variations comparable to noise currents and means responsive to electrical variations of a value larger than noise currents for reducing said bias to permit said device to substantially translate said electrical variations in I ELLSWQR'I'H D. COOK. 

